Actually, the Iraqis were able to put on a pretty healthy and effective insurgency for 5 or 6 years. The US military had to completely invent brand new COIN tactics to fight what they were presented with in that insurgency. From the Iraqi perspective, their insurgency "worked", as the foreign power that invaded is no longer there occupying.
Nobody would ever mistake a rag-tag patchwork of civlian US firearm owners for a trained and commanded military fighting force that would ever take on an invading army force-on-force. That same armed civilian populace, however, could put on insurgent and guerilla warfare very easily.
Part of what I wanted to say except he said it better.
The shear number of arms in the US would enable whoever wanted to fight to be armed. I'm pretty sure the Iraqis did not have 89 guns per 100 people.